feat(mdbook): allow default configuration

[tomatotomata]

Summary

• make the book attribute optional for the mdbook rule
• generate an empty book.toml so mdBook uses its documented default configuration
• add a build test covering a book with only src/SUMMARY.md and chapter sources

This addresses #4194. The issue discussion asked for a PR that demonstrates mdBook can run without a user-provided book.toml; an empty configuration parses to mdBook's defaults, including src as the source directory. I kept the generated file in the package staging tree so the existing input mapping and mdbook_server provider continue to use the same config path. Let me know what you think.

Validation

• git diff --check passed
• inspected mdBook 0.4.44 configuration defaults: an empty TOML file loads src as the source directory
• Bazel execution was not available in this Windows environment, so the new build target is left for the repository CI matrix

[UebelAndre]

Thanks! I had just one question

[tomatotomata]

Yes, that makes sense. I removed the provider-specific path and now every staged source, including the generated default config, uses _src_dest_path(file). The focused mdBook tests for default config, simple, stitched, and bzlmod pass on 29948fbcd; the broader external-source build still hits the existing Windows path lookup failure. The updated branch is pushed, so the simpler interface is in place.

[UebelAndre]

This is looking good, once CI passes I'm happy to merge!

[tomatotomata]

Buildkite and the formatting check are green on 813abe1, and the Google CLA check is green too. I dropped the provider-specific field as you suggested, so the generated default config now goes through the same _src_dest_path() path as the other staged sources. Let me know if anything else looks off.

[UebelAndre]

Thank you!